Method and apparatus for abrading



Nov, 22, 1966 s. l. ASHWORTH 3,286,406

METHOD AND APPARATUS FOR ABRADING Filed Feb. 26, 1964 2 Sheets-Sheet 1 Nov. 22, 1966 s. 1. ASHWORTH 3,236,406

A METHOD AND APPARATUS FOR ABRADING Filed Feb. 26, 1964 2 Sheets-Sheet 2 United States Patent Office 3,286,406 Patented Nov. 22, 1966 3,286,406 METHOD AND APPARATUS FDR ABRADING Stewart Ives Asliworth, Crimond, Outhill, near Studley, England, assignor to Abrasive Developments Limited, Solihnll, England, a British company Filed Feb. 26, 1964, Ser. No. 347,588 Claims priority, application Great Britain, Feb. 27, 1963, 7,808/63; Nov. 15, 1963, 45,178/63 19 Claims. (Cl. 518) This invention relates to abrading and particularly to methods of and apparatus for abrading with dry particles.

The usual method of abrading a surface with abrasive particles is to direct against the surface a jet of compressed air carrying the particles. It has also been proposed to supply additional suction to that surface over a confined area to draw off the detritus and spent abrasive while at the same time permitting in-flow of air to the abraded area from the surrounding atmosphere.

An object of the present invention is to provide a method of and apparatus for abrading which does not involve the use of compressed air. The avoidance of the use of compressed air has the advantages of avoiding or reducing the tendency for scattering of detritus and abrasive particles from the part of the surface being abraded and also of economy in cost since expensive air compressors can be dispensed with.

According to one aspect of the invention, a method of abrading a surface on a workpiece comp-rises the steps of forming an enclosure with said surface as a wall thereof, applying suction to said enclosure, inducing the flow of primary air into the enclosure by the removal of the air in the enclosure by suction while restricting the entry of secondary air from the surrounding atmosphere into the enclosure around the periphery of said surface, entraining abrasive in said primary air prior to the latter entering the enclosure, directing the abrasive entering the enclosure against said surface and entraining the spent abrasive in the air being removed from the enclosure by suction.

Conveniently, the primary air is caused to flow through the enclosure in a direction having a component generally parallel to said surface. The primary air may enter the enclosure in a direction generally normal to said surface prior to flowing in the direction having a component generally parallel to the surface.

The method is applicable to the abrading of the internal surface of a hollow workpiece. In such an application, the surface which forms a wall of the enclosure is said internal surface. The workpiece may be tubular and in such circumstances suction is applied to one end of the bore of the tubular workpiece and the primary air and abrasive enter at the other end of said bore.

In another application of the method, this can be used for abrading the external surface of an elongated workpiece such as a pipe or Wire. Insuch an application, the external surface of the elongated workpiece forms the inner wall of a tubular enclosure and the abrasive is directed against the surface from several directions simultaneously. In one such arrangement, the directions from which the external surface is abraded are normal to a helix and intersect the helix at spaced positions thereon.

According to another aspect of the invention, apparatus for abrading a surface on a workpiece comprises a blasting chamber, a primary air inlet conduit connected at one end to the blasting chamber, means for entraining abrasive in the primary air flowing through the conduit upstream of its entry into the blasting chamber, aperture means in the wall of the chamber to be closed by said workpiece, means for applying suction to said chamber to draw primary air and abrasive into the chamber and remove air and spent abrasive from the chamber when said aperture means is closed by the workpiece, means for directing abrasive entering the chamber against said surface and means for restricting the entry of secondary air into the chamber around the periphery of the aperture means when the latter is closed by the workpiece.

. Preferably the abrasive entraining means includes metering means adjustable to vary the quantity of abrasive entrained in the primary air. The metering means may include a recess in the lower wall of the inlet conduit and a vertically adjustable metering tube extending into the recess, the inlet conduit may also include a bafile up-stream of the recess to cause the primary air flowing through the conduit to enter the recess and entrain the abrasive therein.

The apparatus may include means for separating .reusable abrasive from the air flow leaving the blasting chamber and interposed between such chamber and the suction means. The metering means may be fed with abrasive from the separating means.

In one embodiment of the invention, the blasting chamber is portable and is connected to the suction means and the abrasive entraining means by flexible pipes.

In another embodiment of the invention the apparatus is adapted for abrading elongated workpieces and comprises a tubular blasting chamber having an entry for the workpiece and an exit therefor and the directing means is arranged to cause the abrasive to impinge upon the workpiece fr-om several directions simultaneously. The restricting means may comprise seals at the entry and the exit of the chamber which seal against the external surface of the workpiece as the latter passes through the chamber. The suction means may be connected to the chamber downstream of the directing means in the direction of travel of the workpiece.

In the arrangement for abrading elongated workpieces, the directing means may comprise a series of nozzles with their outlets arranged on the periphery of a circle. Alternatively, the directing means may comprise a series of nozzles with their outlets arranged along a helix which surrounds the workpiece.

According to a further aspect of the invention, apparatus for abrading the internal surface of a tubular workpiece comprises a blasting chamber formed by the bore of the workpiece and two end members therefor, a primary air inlet conduit connected to one of said end members, means forentraining abrasive in the primary air flowing through the conduit up-stream of its entry into the blasting chamber, means, connected to the other of said end members, for applying suction to said chamber to draw primary air and abrasive into the chamber and remove air and spent abrasive from the chamber when the ends of the bore are closed by said end members, means on said one end member to direct abrasive entering the bore against the wall thereof and means for restricting the entry of secondary air into the bore around the peripheries of the end members when the latter close the ends of the bore.

Several embodiments of the invention will now be described in detail by way of example with reference to the accompanying drawings in which:

FIGURE 1 is a diagram showing the general arrangement of a first embodiment of the apparatus which includes a portable blasting chamber;

FIGURE 2 is a diagram showing an arrangement which may be substituted for the blasting chamber in FIGURE 1 for abrading the internal surfaces of tubular workpieces;

FIGURES 3 and 4 are end and side elevations respectively of an arrangement which may be substituted for the blasting chamber of FIGURE 1 for abrading the external surfaces of elongated workpieces; and

FIGURES 5 and 6 are end and side elevations respectively of an alternative arrangement which may be substituted for the blasting chamber of FIGURE 1 for abrading the external surfaces of elongated workpieces.

Referring now to FIGURE 1, the abrading apparatus there shown comprises a portable blasting chamber 10 which presents at one face an aperture 11 adapted to be placed on a fiat surface of a workpiece 12 with the edge portions of the aperture close to the surface. The edges of the aperture are provided with seals 13 which may comprise rollers, rubber or plastic shielding or bristles or a combination of any of these elements and which restrict the entry of secondary air in the chamber from the surrounding atmosphere as will hereinafter be described.

Into the top of the chamber 10 there extends directing means in the form of an accelerating nozzle 14 which is attached to the end of a flexible portion 15 of a primary air inlet conduit indicated generally at 16. The other end of the conduit 16 is provided with a bell-mouth 17 which constitutes an air intake and is open to atmosphere. Between the air intake 17 and the nozzle 14 there is provided in the conduit 16 a recess 18 in the lower wall thereof and a baffle 19 is provided up-stream of the recess 18 so that air flowing through the conduit from the air intake 17 to the nozzle 14 is forced to flow through the recess 18.

The upper end of the blasting chamber 10 is also connected to one end of a suction pipe 20 which at its other end is connected to the inlet 21 of a first cyclone separator 22. The cyclone is of conventional form having a rubber lining and is deliberately made relatively ineffective so that only the heavier particles contained in the air stream entering the cyclone are separated and fall to the bottom of the cyclone, the lighter particles being carried out of the top outlet 23 of the cyclone into a second cyclone 24. The cyclone 24 is of a higher reentrant type which operates under a fairly high vacuum and effectively separates out the finer particles. The effectiveness of the second cyclone is believed to be due not only to the low pressure within the cyclone but also because the low pressure leads to adiabatic cooling within the central spinning whirl of the cyclone so that any moisture in the air stream condenses on the dust particles thus increasing their weight and increasing the separating effect of the cyclone. This theory is supported by the fact that if the dust is inspected immediately after being separated by the second cyclone 24 it is slightly damp and is at a low temperature.

The lower outlet 25 from the cyclone 24 leads to an intermediate box 26 which in turn leads to a dust collecting box 27. As will be seen from the drawing, the intermediate box 26 is of such a shape that the inlet 28 to the dust collecting box is off-set from the vertical axis 7 of the cyclone 24. This prevents the whirling air in the cyclone 24 from disturbing the dust in the dust collecting box 27.

The cyclone 24 has an upper outlet 29 which is connected through an adjustable valve 30 to suction means constituted by a multi-stage centrifugal fan 31. The valve 30 regulates the intensity of the vacuum created within the cyclone and therefore controls the entry of air into the intake 17.

The first cyclone 22 has a lower outlet 32 which is connected to the upper end of an upper hopper 33. The upper hopper in turn has a lower outlet 34 which discharges into a middle hopper 35. The middle hopper 35 has an outlet 36 which discharges into a lower hopper 37 and the lower hopper 37 has an outlet 38 which discharges into a metering tube 39 whose lower end is received in the recess 18 in the conduit 16.

Communicating with the middle hopper is a pipe 40 which contains first valve means 41 and which terminates in a bell-mouth 42 which is open to atmosphere. The valve means 41 is driven by cycling means so that the valve means is alternately openend and closed. The pipe 40 is connected by a pipe 43 to the inlet 21 to the first cyclone 22. A restrictor 43a is inserted between the pipe 41 and the pipe 43.

At the outlet 34 of the upper hopper is provided second valve means 44 in the form of a flap valve. The valve is freely hinged and is operated by differences between the pressures existing in the upper and middle hoppers. When the pressure in the middle hopper is greater than the pressure in the upper hopper the second valve means will close. Conversely, when the pressure in the upper hopper is greater than the pressure in the midde hopper, the second valve means will open.

Third valve means 45 are provided at the outlet 36 of the middle hopper and these valve means operate in a manner similar to the second valve means 44. Thus when the pressure in the lower hopper is greater than the pressure in the middle hopper the third valve means 45 will close whereas when the pressure in the middle hopper is greater than the pressure in the lower hopper, the third valve means will open.

It is apparent that any abrasive stored above either of the valve means 44 or 4 5 will tend to cause them. to open even when the pressure below the valve means is slightly greater than the pressure above the valve means. However, the pressure differences are arranged to take into account the weight of such abrasive and to maintain the valve means closed when required.

In operation, the centrifugal pump 31 is set in operation and the valve 30 is opened to induce suction in the suction pip-e as :far as the blasting chamber 10. When the blasting chamber 10 is in contact with the workpiece 12 so that the seals 13 substantially seal the aperture 11 to the workpiece, the suction is applied to the conduit 16 and primary air starts to flow through the conduit 16 from the air intake 17 to the nozzle 14. As the air flows along the conduit 16 it will be deflected by the battle :19 to flow through the recess 18. As will hereinafter be described, abrasive is delivered into the recess 18 through the metering tube 3 9. The abrasive is entrained in the primary air stream and passes with the air along the conduit 16 to the accelerating nozzle 14. The abrasive particles being accelerated .inthe nozzle will impinge upon the surface of the workpiece 12 and will abrade the surface thereof. The spent abrasive and air will then pass along the suction pipe 21 to the inlet 21 of the first cyclone 22.

The seal 13 restricts the entry of secondary air into the blasting chamber 10 from the surrounding atmosphere. The degree of restriction will depend upon the form of the seal and may be complete, if the seals are in the form of a rubber or similar skirt, or incomplete if the seals are in the form of bristles. In the latter case secondary air will enter the chamber through the seals and will leave it with the primary air through the suction pipe 20.

Since the cyclone '22 is somewhat ineffective it is thus able to separate the reusable abrasive which has not been shattered from that which has been shattered into dust. The reusable abrasive passes to the bottom of the first cyclone 22 into the upper hopper 33.

The shattered abrasive and the detritus from the surface of the workpiece remain in the air stream and pass through the outlet 23 of the first cyclone 22 to the second cyclone 24. In this cyclone, the shattered abrasive and detritus are separated from the air and the latter passes through the outlet from the cyclone through the valve 30 to the centrifugal fan 31. The dust, comp-rising shattered abrasive and detritus falls to the outlet 25 of the second cyclone 24, passes into the intermediate box 26 and thus into the dust collecting box 27.

Returning now to the reusable abrasive, this collects in the upper hopper 33. Assuming that the first valve means 41 is in a position such that the middle hopper 35 is in communication with atmosphere, the pressure in the middle hopper will be greater than the pressure in the upper hopper, the latter pressure being equal to the pressure in the cyclone 22. The valve 44 will thus be shut and abrasive will build up in the upper hopper 33. The lower hopper 37 is always at atmospheric pressure by virtue of apertures 46 in the wall thereof. It follows that, when the pressure in the middle hopper is atmosp'heric, any abrasive in the middle hopper above the third valve means 45 will open the valve means since the air pressure is the same on both sides thereof and the abrasive will [fall into the lower hopper '37 and from thence to the metering tube 39 and the conduit 16.

If now the first valve means 41 moves to a position such that the middle hopper 35 is in communication with the cyclone inlet 21, the air pressure on both sides of the second valve means 44 will be equal and any abrasive in the upper hopper will cause the second valve means 44 to open and the abrasive will pass to the middle hopper 35. In this situation since the middle hopper is at a lower pressure than atmospheric, the third valve means 45 will be shut so that the abrasive delivered to the middle hopper will remain therein until the first valve means again places the middle hopper in communication with atmosphere. The first valve means are arranged to cycle at a predetermined rate so that there is a constant supply of abrasive being passed from the cyclone 22 to the metering tube 39 through the upper, middle and lower hoppers.

By way of example, the timing of the operation of the first valve means may be arranged so that there will be no buildup of abrasive in either the upper or middle hoppers. For example, the first valve means may be arranged to be open for ten seconds followed by a closed period of ten seconds followed by an open period of ten seconds and so on.

So long as the blasting chamber is sealed to the surface of the workpiece 12 by means of the seal 13, the apparatus will operate as described. The pressures used are such that the blasting chamber may be moved over the surface of the workpiece while the suction means is in operation thus to abrade an area of the workpiece. If, however, the blasting chamber is lifted off the surface of the workpiece so that the aperture therein is open to atmosphere, the interior of the blasting chamber will be at atmospheric pressure. Consequently, there will be no suction in the inlet conduit 16 and the feeding of abrasive particles to the blasting chamber will be automatically cut-off. This has the :great advantage that as soon as the blasting chamber is lifted from the workpiece there will be no more abrasive discharged so that a workman cannot inadvertently distribute abrasive all over the workshop as is sometimes possible with apparatus in the abrasive is entrained in compressed air. The apparatus will again be put into operation by bringing the seal 13 into contact with the surface of the workpiece so that suction is applied to the inlet conduit :16 and abrasive is entrained in the primary air stream and directed by the nozzle 14 against the surface to be abraded. It will be appreciated that if the surface of the workpiece to be treated is not plane, the aperture in the blasting chamber and the seal around the aperture may be shaped to conform to the shape of the surface to be abraded. Alternatively, or in addition, the seal may be made s-ufliciently deformable to conform to and fit closely to the surface of the workpiece.

Referring now to FIGURE 2, this shows diagrammatically apparatus for abrading the internal surface of a tubular workpiece 50. The apparatus includes two end members 51 and 52 which are arranged to embrace the ends of the tubular workpiece 50 in an airtight manner. The end pieces 51 and 52 are arranged to be clamped in position by means of clamps 53 which extend along the length of the workpiece. The one end member 51 is connected to the primary air inlet conduit 16 and the other end member 52 is connected to the suction pipe 20.

When the device is in operation, suction applied to the suction pipe 20 evacuates the bore of the workpiece 50 thus drawing primary air and abrasive in through the inlet conduit 16 to the bore of the workpiece 50. The end member 51 is provided with a nozzle 54 which ensures that the abrasive is caused to impinge upon the wall of the tubular workpiece and thus abrade the same.

Turning now to FIGURES 3 and 4, these show apparatus for abrading the external surface of a cylindrical tube. The apparatus comprises a cylindrical chamber 55 of diameter approximately twice that of the tube 56 to be abraded. An entry 57 for the tube is formed with one circular end of the chamber 55 and an aligned exit 58 is formed at the other end. The air entry and exit are furnished with seals 60 which seal against the external surface of the tube 56 when the latter extends through the entry and exit.

Extending radially from the chamber adjacent to the exit is an outlet 61 which is arranged to be connected to the suction pipe 20.

Radiating from the chamber 55 adjacent to the entry 57 are six radial pipes 62 whose outer ends are joined by a circular manifold 63. The manifold is provided with an inlet for air and abrasive midway between the ends of two of the adjacent tubes 62. Mounted in each tube 62 is a nozzle 64 having a main cylindrical portion 65 of smaller diameter than the tube 62 and a convergent portion 66 at its outer end extending between the outer end of the cylindrical portion 65 and the inner wall of the tube 62. The inner ends of the nozzles project into the chamber 55 and are directed towards the centre thereof so that abrasive and air passing through the nozzles will be caused to impinge upon the tube 56 from several directions simultaneously.

The operation of the apparatus is as follows, the tube 56 is located in the chamber passing through the entry and exit 57 and 58 thereof and the sealing devices 59 and 60 seal against the outer surface of the tube. Suction is then applied to the suction pipe 20, which is connected to the outlet 61, and as a result of evacuation of the chamber 55, primary air and abrasive is drawn through the inlet conduit 16 into the manifold 63 and then passes through the tubes 62 and the nozzles 64 to impinge upon the outer surface of the tube 56. Spent abrasive, detritus and air are continuously removed from the chamber 55 through the outlet 61. The material removed from the chamber then passes through the cyclone separators 22 and 24 as described above and the reusable abrasive is returned to the inlet conduit 16.

It will be seen that as the tube 56 is moved longitudinally through the chamber, each part of the tube will be subjected to abrasion from several directions simultaneously so that the tube need not be rotated to obtain complete abrasion of its external surface.

Referring now to FIGURES 5 and 6, the apparatus thereshown is a modification of that shown in FIGURES 3 and 4. The main difference is that in FIGURES 5 and 6 the tubes with their nozzles are arranged so that the outlets lie upon the path of a helix which is co-axial with the axis of the chamber.

pipes 71 extend generally radially from the chamber wall and these pipes have their outlets upon a helix which is co-axial with the axis of the chamber. Three of the pipes 71 are connected by flexible conduits 72 to a manifold 73 whereas the other three pipes 71 are connected by flexible conduits 74 to a manifold 75. The manifolds 73 and 75 are connected in turn to the inlet conduit 16.

The chamber also has connected thereto a suction outlet 76 which is to be connected to the suction pipe 20.

The operation of the embodiment of FIGURES and 6 is similar to that of the embodiment of FIGURES 3 and 4. When suction is applied to the suction outlet 76, primary air is caused to flow along the inlet conduit 16 and passes to the manifolds 73 and 75 and thus to the tubes 71. The tubes contain nozzles as described with reference to FIGURES 3 and 4 and the air and abrasive is caused to impinge against the outer surface of the tube 70.

The embodiment shown in FIGURES 3 and 4 and the embodiment shown in FIGURES 5 and 6 both have the advantage that, when the tube is removed from the chamber, the suction pipe 20 will draw air through the entry and exit of the chamber and no suction will be applied to the inlet conduit 16 and therefore there will be no flow of primary air with its entrained abrasive. It follows that as soon as the tube is removed from the chamber abrasive will no longer flow into the chamber so that there will be no danger of abrasive escaping from the chamber into the surrounding atmosphere.

The methods and apparatus described above have the following advantages.

(a) By using a large volume of air and abrasive at a low pressure, it is possible to obtain a better process finish with less smashing of the abrasive particles and the avoidance of imbedding the abrasive particles in the surface of the work as occurs in machines using compressed air.

(b) The blast pattern of the abrasive and air leaving the nozzle follows the contour of the nozzle outlet which can be adapted to fit articles of awkward shapes.

(c) It is possible to scale the apparatus up to have an extremely large capacity and by using a nozzle in the form of a long slit it is possible to use the apparatus for blasting large flat sheets.

((1) The abrasive remains clean due to the separating system.

(e) Unlike the compressed air system which uses venturi nozzles, the maximum translation of air pressure into velocity of abrasive takes place.

Reference is made to co-pending application Serial No. 347,589, filed February 26, 1964, in which is described and claimed an abrading machine having the separating system described with reference to FIGURE 1 of the drawings accompanying the present application.

What I then claim is:

1. A method of abrading a surface on a workpiece comprising the steps of forming an enclosure with said surface as a wall thereof, applying suction to said enclosure, inducing the flow of primary air into the enclosure by the removal of the air from the enclosure by suction while restricting the entry of secondary air from the surrounding atmosphere into the enclosure around the periphery of said surface, entraining abrasive in said primary air prior to the latter entering the enclosure, directing the abrasive entering the enclosure against said surface and entraining spent abrasive and detritus in the air being removed from the enclosure by suction.

2. A method according to claim 1 wherein the primary air is caused to flow through the enclosure in a direction having a component generally parallel to said surface.

3. A method according to claim 2, wherein the primary air enters the enclosure in a direction generally normal to said surface.

4. A method according to claim 1, wherein said surface is the internal surface of a hollow workpiece.

5. A method according to claim 4, wherein the workpiece is tubular and wherein suction is applied to one end of the bore of the workpiece and the primary air and abrasive enter at the other end of said bore.

6. A method according to claim 1 wherein said surface is the external surface of an elongated workpiece and forms the inner wall of a tubular enclosure and wherein the abrasive is directed against said surface from several directions simultaneously.

7. A method according to claim 6, wherein said directions are normal to a helix and intersect the helix at spaced positions thereon.

8. Apparatus for abrading a surface on a workpiece comprising a blasting chamber, a primary air inlet conduit connected at one end to the blasting chamber,- means for entraining abrasive in the primary air flowing through the conduit up-stream of its entry into the blasting chamber, aperture means in the wall of the chamber to be closed by said workpiece, means for applying suction to said chamber to draw primary air and abrasive into the chamber and remove air and spent abrasive from the chamber when said aperture means is closed by the workpiece, means for directing abrasive entering the chamber against said surface and means for restricting the entry of secondary air into the chamber around the periphery of the aperture means when the latter is closed by the workpiece.

9. Apparatus according to claim 8, including metering means forming part of the abrasive entraining means and adjustable to vary the quantity of abrasive entrained in the primary air.

10. Apparatus according to claim 9, including a vertically adjustable metering tube extending into a recess in the inlet conduit and a bafile in the inlet conduit upstream of the recess to cause the primary air flowing through the conduit to enter the recess.

11. Apparatus according to claim 8, including means for separating reusable abrasive from the air flow leaving the blasting chamber and interposed between such chamber and the suction means.

12. Apparatus according to claim 11, including metermg means fed with abrasive from the separating means and adjustable to vary the quantity of abrasive entrained in the primary air.

13. Apparatus according to claim 8, wherein the blasting chamber is portable, the apparatus including flexible p1pes interconnecting the blasting chamber to the suction means and the abrasive entraining means.

14. Apparatus according to claim 8, wherein the apparatus is adapted for abrading elongated workpiece and comprises a tubular blasting chamber having an entry for the workpiece and an exit therefor and wherein the directing means causes the abrasive to impinge upon the workpiece from several directions simultaneously.

15. Apparatus according to claim 14 wherein the restricting means comprise seals at the entry and the exit of the chamber which seal against the external surface of the workpiece as the latter passes through the chamber.

16. Apparatus according to claim 14, wherein the suction means is connected to the chamber downstream of the directing means in the direction of travel of the workpiece.

17. Apparatus according to claim 14, including a series of nozzles with their outlets arranged on the periphery of a circle and comprising the directing means.

18. Apparatus according to claim 14 including a series of nozzles with their outlets arranged along a helix surrounding the workpiece and comprising the directing 7 means.

19. Apparatus for abrading the internal surface of a tubular workpiece comprising a blasting chamber formed by the bore of the workpiece, two end members for said bore, a primary air inlet conduit connected to one of said .end members, means for entraining abrasive in the primary air flowing through the conduit upstream of its entry into the blasting chamber, means connected to the other of said end members, for applying suction to said chamber to draw primary air and abrasive into the chamber and remove air and spent abrasive from the chamber when the ends of the bore are closed by said end members, means on said one end member to direct abrasive entering the bore against the wall thereof and means for restricting the entry of secondary air into the bore around the peripheries of the end members When the latter close the ends of the bore.

References Cited by the Examiner UNITED STATES PATENTS LESTER M. SWINGLE, Primary Examiner. 

8. APPARATUS FOR ABRADING A SURFACE ON A WORKPIECE COMPRISING A BLASTING CHAMBER, A PRIMARY AIR INLET CONDUIT CONNECTED AT ONE END TO THE BLASTING CHAMBER, MEANS FOR ENTRAINING ABRASIVE IN THE PRIMARY AIR FLOWING THROUGH THE CONDUIT UP-STREAM OF ITS ENTRY INTO THE BLASTING CHAMBER, APERTURE MEANS IN THE WALL OF THE CHAMBER TO BE CLOSED BY SAID WORKPIECE, MEANS FOR APPLYING SUCTION TO SAID CHAMBER TO DRAW PRIMARY AIR AND ABRASIVE INTO THE CHAMBER AND REMOVE AIR AND SPENT ABRASIVE FROM THE CHAMBER WHEN SAID APERTURE MEANS IS CLOSED BY THE WORKPIECE, MEANS FOR DIRECTING ABRASIVE ENTERING THE CHAMBER AGAINST SAID SURFACE AND MEANS FOR RESTRICTING THE ENTRY OF SECONDARY AIR INTO THE CHAMBER AROUND THE PERIPHERY OF THE APERTURE MEANS WHEN THE LATTER IS CLOSED BY THE WORKPIECE. 